The present invention relates to a method of making safe a heat treatment enclosure operating under a controlled atmosphere. It relates more particularly to a method of making safe a treatment enclosure operating under an atmosphere having a high hydrogen content.
According to another aspect of the invention, it also relates to a heat treatment plant, especially one such as the enclosures encountered in the field of vertical furnaces for the continuous heat treatment or coating of metal strip.
Continuous heat treatment furnaces, such as for example annealing or coating furnaces, are composed of one or more of the following zones: preheating, heating, soaking, slow-cooling and rapid cooling.
Each of these zones makes it possible to heat the strip, maintain its temperature or cool it with precise heating and cooling rates and for precise times defined by the metallurgical treatment cycle corresponding to the material to be treated.
New grades of steel recently developed have required new treatment cycles to be defined with new heating and cooling rates.
The high cooling rates dictated by these new cycles are obtained by increasing the pressure of the cooling gas blown onto the strip or by optimizing the blowing geometry so as to obtain the highest possible gas/strip exchange coefficients. The equipment used at the present time with low hydrogen contents show that the maximum rates that can be achieved in cooling are about 60xc2x0 C./second for a strip 0.8 mm in thickness, above which rates the dimensions, the power absorbed by the fans and the cost of the plant become too great or the excessive gas blowing velocities cause instability in the position of the strip. To achieve these cooling rates it is also possible to limit the velocity of the line, but this correspondingly limits its production and makes this technique unprofitable.
To increase the performance in the cooling zones requires the use of an atmosphere with a high hydrogen content, up to 100%, so as to obtain a large increase in the exchange coefficient capable of generating cooling rates of about 80 to 200xc2x0 C./second or more, for a strip 0.8 mm in thickness, these being compatible with the treatment cycles to be obtained at the nominal line speed.
The method of control allowing the percentage concentration of hydrogen to be increased to such values poses a major problem owing to the nature of this gas (especially the explosive character of this gas); in particular, it is impossible to use such an atmosphere throughout the line where contact with air in the strip entry and exit zones would entail major risks for the operating safety of the plant. It is therefore necessary to limit the zone operating with a high hydrogen content to the rapid cooling zone and to isolate this zone from the adjacent zones of the line where a reduced hydrogen content is sufficient for the treatment process and to ensure the operating safety of the line.
This operating safety must be ensured:
during nominal or stabilized operation of the line;
during changes to the line speed or to the strip format or when incidents occur, such as for example line shutdown or strip breakage.
These various line operating conditions result in pressure variations in the chambers, variations which are amplified by the considerable expansion or contraction of the atmosphere containing a large amount of hydrogen, causing an atmosphere to flow from one chamber to another. This changes the composition of the atmosphere and may entail major risks if the zone with a high hydrogen content is not controlled.
The operating method allowing such cooling or heating rate gradients to be obtained forms the subject matter of French Patent Application No. 2 746 112 filed by the Applicant.
The solutions provided by the method forming the subject of the invention and the plant allowing such a method to be implemented offer a response to these safety problems, while reconciling requirements relating to atmosphere exchange control. These requirements also make it possible to optimize the operation of the heat treatment plant and to reduce the consumption of the controlled atmosphere (especially hydrogen) during nominal operation of this line or when there is a change in its speed or in the format of the strip to be treated, or when there is a production incident such as, for example, line shutdown or strip breakage.
For this purpose, the method of making safe a heat treatment enclosure operating under a gas atmosphere, the said enclosure comprising a chamber for rapidly cooling a metal strip running from an upstream chamber to a downstream chamber by means of a plurality of guide rollers, is characterized in that the said strip is confined within the rapid cooling enclosure with the aid of at least one pressure-balancing duct and of a plurality of gas locks placed between the various chambers, and in that the pressures of the gas atmospheres between the chambers are balanced by means of ducts, by controlling the flow rates of the gas flowing through the said gas locks.
Further features and advantages of the present invention will become apparent from the description given below with reference to the appended drawings which show illustrative embodiments thereof, these being devoid of any limiting character. In the figures: